Automatic transmissions have been used in different types of motor vehicles for a long time. While the driver has to carry out multiple chronologically coordinated and situation-adapted operations with manually shifted transmissions, automatic transmissions relieve the driver of these operations, allowing him to concentrate better on traffic situations.
An experienced driver will certainly be able to arrange the chronological shift arrangement of the main clutch and accelerator pedals, the time and speed of actuation of the gear selection lever and the force acting on the gear selection lever in a plurality of ways, and optimize them according to his respective intention. For example, if he desires an especially comfortable and shock-free gear selection, he will actuate the main clutch pedal rather slowly, after having sensitively released the accelerator pedal. In this situation, he will actuate the gearshift lever only after a short pause, after disengaging the main clutch and avoid excessively rapid shift movements or excessive pedal force. Conventional automatic gearshift controls are frequently based on such shift sequences in order to provide the passengers with high shifting and driving comfort, as well as the possibility of shifting with as little wear as possible.
However, this shifting strategy has the disadvantage that during the shifting operation the traction is interrupted for a relatively long time. Particularly, when high traction is desired, while operating uphill with trailers, for instance, or in normal operation on ramps and other steep gradients, this may lead to the vehicle losing so much speed during this traction interruption, that the originally selected gear can no longer be shifted or, in any case, is no longer the optimum gear. Moreover, even at the desired high vehicle acceleration performance, the negative effect of traction interruption becomes evident. In any case, a shifting strategy of this type impairs the potential driving performance of a vehicle compared with a vehicle with manual transmission and a driver who shifts fast and precisely. Last but not least, this has contributed to the problems in accepting automatic transmissions by some sports-minded drivers.
From DE 101 32 738 A1, a method for controlling gear disengagement in an automatic multi-step transmission is known, in which the drive motor torque is reduced during a gear change. Depending on the vehicle instantaneous acceleration, the main clutch is released when a first acceleration threshold value is reached and the gear is disengaged when a second acceleration threshold value is reached. In the process, the first and second acceleration threshold values are selected and/or chosen independently of one another and, depending on the original acceleration of the vehicle, the currently engaged gear and the injection volume of the motor.
If the first threshold value is reached, once a signal to release the main clutch has been given, a signal to disengage the gear is issued immediately. If the second threshold value has a higher absolute value than the first threshold value, and hence the second threshold value is reached with a drop in acceleration of the vehicle, the signal to disengage the gear will be issued at this moment, while the signal to release the main clutch will only be delivered later, i.e., when the first acceleration threshold value is reached.
The known method is based on the consideration that gear disengagement, independently of the position of the main clutch, is only possible if a relatively low actuating force is sufficient for this purpose. It is apparent from the exemplary embodiment that, at this point, the volume of the fuel injected into the drive motor has long been reduced to zero and hence the still remaining acceleration of the vehicle is essentially due to mass inertia and elastic deformation of elements of the power train and resultant torque at the input side of the transmission. Significant acceleration of the transmission due to early gear disengagement is, therefore, unlikely and not intended in any case.
Thus, DE 101 32 738 A1 provides a valuable approach for the reduction of traction interruption in automatic multi-step transmissions with regard to gear disengagement, but it is based on continuous determination and evaluation of the vehicle acceleration and in no way addresses the problem of engaging a selected target gear as fast and effectively as possible.
Against this background, the object of the present invention is to present a control method for an automatic transmission which allows a gear change and, in particular, downshifting with minimum traction interruption.